I am trying to add several custom fields that already exist on the classic UI version of the “Create Payment” popup into the Modern UI panel of the same screen.

However, I am running into two issues:
1. When I add the fields directly in my Modern UI extension:
I get an error:
“ID does not exist”
Even though these fields exist in the DAC and work correctly in Classic UI.

2. When I try to modify the existing Modern UI extension file (SO301000_CreatePayment.html):
The customization works, but when I include this file inside a Customization Project, I get:
“Duplicate ID”
or
“ID already defined in another extension”. This makes the whole customization fail.
I suspect the issue happens because my customized Modern UI file is being added into frontendSources, which already contains the default SO301000_CreatePayment.html.
Because of that, the same element IDs exist twice, and this results in the duplicate ID error.
However, I am not sure what the correct approach is in this situation:
-
Should I replace the existing file?
-
Should I create a separate extension layer?
-
Or is there a recommended way to add new fields to a Modern UI popup without generating duplicate IDs?